1. Field of the Invention
The present invention relates generally to the field of batteries and, more specifically, to the separators which are positioned between the positive and negative plates of batteries, a method for producing such separators and a method for assembling a cell using such separators.
2. Definitions
Subsequently herein, the term "percent v/v" means percent by volume; the term "percent w/w" means percent by weight; all temperatures are in .degree.C.; and the following abbreviations have the meanings indicated: psi means pounds per square inch, .mu. means micron or microns; mg=milligram or milligrams; g=gram or grams; kg=kilogram or kilograms; l=liter or liters; ml=milliliter or milliliters; m=meter or meters; and cm=centimeter or centimeters. Unless the context indicates otherwise, references in the following specification and the appended claims to comparative thicknesses of separators should be understood to mean the thickness of such separators under nominal, substantially identical, pressures because, as is known to those skilled in the art, one cannot obtain a meaningful thickness measurement of glass fiber separator materials under no pressure; only when some pressure is applied does a thickness measurement have some repeatability and reliability. As used herein and in the appended claims, the phrase "substantially binder free" is to be understood to refer to glass fiber webs or sheets in which the primary mechanism for holding the sheet or web together is the entanglement of individual glass fibers as opposed to an adhesive binder mechanism such as that obtained by the addition of methyl cellulose to glass fibers. The phrase "substantially binder free" is not meant to exclude glass fiber webs or sheets which include materials which contribute to some minor degree of ionic bonding, for example, glass fiber webs or sheets which contain sodium sulfate which suppresses dendritic growth but which also contributes to minor ionic binding of the fibers; fibers in such a sheet or web would still be bound together primarily by fiber entanglement and the web or sheet itself would be substantially binder free. Similarly, references to a resiliency suppressing liquid which is "substantially free of a binder" is meant to refer to liquids which do not contain adhesive binders or binders which, if added to a glass fiber sheet or web, would replace the entanglement of individual fibers as the primary binder mechanism in the web. That phrase is not meant to exclude liquids which contain, for example, sodium sulfate or other materials which would contribute a minor degree of ionic bonding.
3. Description of the Prior Art
Valve regulated ("sealed") lead acid (VRLA) batteries are known and they can comprise a plurality of positive and negative plates, as in a prismatic cell, or layers of separator and electrode tightly wound together, as in a "jelly roll" cell. The plates are arranged so that they alternate, negative - positive - negative, etc., with separator material separating each plate from adjacent plates. The separator, typically composed of a mat of glass fibers, serves many purposes. A primary purpose of the separator is to electrically insulate one plate from the other. In addition, in VRLA batteries, glass fiber separator material provides innumerable gas channels between the plates through which oxygen can migrate from the positive electrode when generated there to the negative electrode where it can be recombined with hydrogen, according to the oxygen cycle.
Glass fiber separator material is produced commercially on paper making equipment. In most cases, no binder is used to make separator sheets; the entanglement of individual fibers serves to maintain the sheet in a cohesive structure. A great deal of work has been directed to modifying the glass fiber furnish to improve battery performance. Some of the work has entailed the addition of synthetic fibers for various reasons. Other work has been directed to the use of binders to releasably maintain glass fiber separator in a compressed state. Recent patents are discussed below.
U.S. Pat. No. 4,465,748 (Harris) discloses glass fiber sheet material for use as a separator in an electrochemical cell, and made from 5 to 35 percent by weight of glass fibers less than 1.mu. in diameter; the patent also discloses a glass fiber sheet for such use wherein there are fibers of a continuous range of fiber diameters and lengths, and most of the fibers are not over 5 mm in length.
U.S. Pat. No. 4,216,280, (Kono et al.), discloses glass fiber sheet material for use as a plate separator in a battery, and made from 50 to 95 percent by weight of glass fibers less than 1.mu. in diameter and 50 to 5 percent by weight of coarser glass fibers. The coarser glass fibers, the reference says, have a fiber diameter larger than 5.mu., preferably larger than 10.mu., and it is advantageous for some of the coarser fibers to have diameters of 10.mu. to 30.mu..
U.S. Pat. No. 4,373,015, (Peters et al.), discloses sheet material for use as a separator in a battery, and "comprising organic polymeric fibers"; both of the examples of the reference describe the sheet material as "short staple fiber polyester matting about 0.3 mm thick", and indicate that the polyester fibers range from about 1.mu. to about 6.mu. in diameter.
Sheet separators for use in conventional (not valve regulated) batteries and comprising both glass fibers and organic fibers are disclosed in all of the following U.S. Pat. Nos.: No. 4,529,677, Bodendorf; No. 4,363,856, Waterhouse; and No. 4,359,511, Strzempko.
U.S. Pat. No. 4,367,271, Hasegawa, discloses storage battery separators composed of acrylic fibrils in an amount of up to about 10 percent by weight, balance glass fibers.
Japanese patent document 55/146,872 discloses a separator material comprising glass fibers (50-85 percent by weight) and organic fibers (50-15 percent by weight).
U.S. Pat. No. 4,245,013, Clegg et al., discloses a separator made by overlaying a first sheet of fibrous material including polyethylene fibers with a second sheet of fibrous material including polyethylene and having a synthetic pulp content higher than the first sheet.
U.S. Pat. No. 4,908,282, Badger, discloses a separator comprising a sheet made from first fibers which impart to the sheet an absorbency greater than 90% and second fibers which impart to the sheet an absorbency less than 80% wherein the first and second fibers are present in such proportions that the sheet has an absorbency of from 75 to 95%. This patent discloses that when this separator is saturated with electrolyte, unfilled voids remain so that gas can transfer from plate to plate for recombination.
It is recognized that, in order to obtain good performance in VRLA batteries, the cell stack comprising the plates and separators must be maintained under compression so that good contact is maintained between the separators and the plates. If the cell stack components reach a condition where they are not under substantial compression, battery failure will occur. In flat plate cells, this compression is conventionally achieved by assembling the cell stack components into a cell stack having a given thickness, physically compressing the cell stack, and inserting the compressed cell stack in a battery case which is sized, relative to the size of the cell stack, to maintain the components of the cell stack under compression. The step of compressing the cell stack is a difficult one because substantial compression is needed. After a cell stack is inserted into a battery case, electrolyte is added. However, because of the compression within a cell stack in a battery case, the rate at which electrolyte is absorbed into the pores of the plates and the separator is very slow. Typically, it takes several minutes to fill a VRLA battery case with electrolyte, even when a vacuum is drawn in the case.
Recently, some effort has been directed to the production of separators which are pre-compressed. European Patent Application 89103907.5 published Sep. 27, 1989 under No. 334091, discloses a dry, glass fiber separator which is pre-compressed and is maintained in that state by means of a water or acid soluble binder, preferably methyl cellulose. According to this patent document, the binder can be mixed in with the glass fiber slurry before it reaches the drying screen of a paper making machine. After most of the liquid in the slurry has been removed from the web, it is compressed between compression rollers. The web leaves the compression rollers at a reduced thickness and the binder adheres the glass fibers together in a compressed state which is maintained by the binder throughout the final drying of the web. The patent document alternatively discloses the application of a water soluble binder to the glass fibers after they have been formed into a web, followed by the steps of compressing and drying.
U.S. Pat. No. 5,091,275 (Brecht et al.) discloses a glass fiber separator which expands when exposed to electrolyte. The separator comprises glass fibers which are impregnated with an aqueous solution of colloidal silica particles and a sulfate salt. The separator is produced by forming a paper making web of glass fibers, impregnating the web with the aqueous mixture of silica and a salt, lightly compressing the impregnated web to remove some of the aqueous solution, partially drying the web, compressing the web to a final thickness and completing the drying of the web. The web is preferably compressed to a thickness which is less than the distance between plates in a given cell, so that insertion of an assembled cell stack into a case is facilitated. When electrolyte is added to the case, the salt dissolves in the electrolyte and the separator expands to provide good contact between the plates and the separators. According to the patent, the silica contributes to the recombination performance of cells incorporating the pre-compressed separator. The silica also contributes a great deal of stiffness to the separator, so much so that the separator may be characterized as rigid.
International patent application PCT/GB80/00159, published Apr. 16, 1981 under No. WO 81/01076, discloses method for assembling a VRLA battery wherein dry separator and plates are assembled into a cell stack, pressure is applied to the cell stack to compress it, electrolyte is added to the compressed cell stack and the pressure is removed but, according to the application, the cell stack maintains its compressed thickness, thereby making it easier to insert the cell stack in the battery case. The amount of electrolyte added is preferably the final amount of electrolyte which will be in the battery.
It should be noted that, in a conventional process for producing glass fiber separator, a glass fiber web is typically passed between compression rollers in the vicinity of the drying screen, in order to produce glass fiber separator of a desired caliper or thickness. This results in an insubstantial thickness reduction of the web at a time when it is still contains a large amount of moisture from the glass fiber slurry from which the web is produced. It is believed that the moisture content of the web when it is compressed in accordance with this conventional technique for controlling its thickness, is such that the ratio of the combined weight of the dry separator and the weight of the moisture to the weight of the dry separator would be between 8:1 and 10:1.